The present invention generally relates to video recording systems and, more particularly, to a versatile digital recording system for recording high resolution video imagery.
Video imagers, i.e., cameras, employing the raster scanning technique are in widespread use ranging from broadcasting to infrared imaging. The raster scanning technique is versatile because it allows imagery to be transmitted at high rates with a single signal channel. The bandwidth of the signal from such a video imager is determined by the number of raster lines that are contained in a video frame. The typical low resolution TV broadcast 525 lines per frame format has a signal band-width of about 5 MHz. The high resolution 1024 lines per frame format has a bandwidth of 17 MHz.
Video imagery that generate imagery with such high or fine resolution of detail are expected to be used with increasing frequency in the future for specialized imaging applications in both military and commercial fields. The common unit used for recording raster scanned video is the analog video recorder (for example, a video cassette recorder, or VCR). Analog recorders are fundamentally limited in bandwidth to less than 6 MHz which is acceptable for recording low resolution imagery, such as standard broadcast 525 lines per frame TV signals. Analog tape recorders are unable to record signals at frequencies higher than this limit due to the inductive nature of magnetic recording heads and the grain size of the magnetic particles on the tape. Video formats with a higher signal bandwidth than 6 MHz cannot be recorded with good fidelity by analog recording systems.
Thus, if an imager's signal exceeds this limit, the resolution of detail in the image will be degraded in the record/reproduce process. Such is the case in a video imager which employs the high resolution 1024 lines per frame format. As explained above, due to bandwidth limitations analog recorders cannot be used for recording high resolution video. Digital recording is an alternative to analog recording which is not subject to the same limitations in bandwidth as analog recording.
Two factors are required to specify the resolution achievable with a digital recording system: (1) the temporal sampling frequency, and (2) the grey shade resolution. The temporal frequency determines the spatial resolution of the image, i.e., the more samples taken per video line the higher the resolution, provided the intrinsic resolution limits of the camera are not exceeded. The temporally sampled units are referred to as pixels, where each pixel is represented by a series of digital bits, known as a word. The maximum sampling rate achievable by an analog-to-digital converter is limited by the pixel word size. In the recent past, for a 10-bit pixel the maximum rate was 40 MHz. This rate represents the upper limit to the bandwidth of a digital recording system. For smaller word sizes the maximum sampling rate is higher, and for larger words the sampling rate is lower than 40 MHz.
The number of grey shades that can be represented by a pixel consisting of b bits is 2.sup.b. Thus, a 10-bit pixel contains 2.sup.10 =1024 steps in its dynamic range. For instance, in a thermal imaging system which has a thermal contrast range of T=40 (i.e. in the image a 0.degree. C. target appears black and a 40.degree. C. target appears white) the smallest thermal contrast that can be measured from the digital data is 40/1024=0.039.degree. C. This is on the order of the minimum resolvable temperature difference (MRTD) of most high quality thermal imaging systems.
Digital recording systems can record video information with a higher bandwidth than analog systems. A digital recorder is capable of recording the imagery from a high resolution imager with full image fidelity. Also, the digital format is compatible with image processing facilities and eliminates the intermediate analog to digital conversion process that is necessary to analyze imagery from an analog recording in an image processor.
However, digital information takes up more space on recording tape than analog information. The temporal rate that information can be recorded on tape thus becomes the important limiting factor in recording high speed video information in digital format. The maximum rate for recording digital information with a tape recorder is determined by the tape drive speed and the packing density of digital bits on the tape. At present, the maximum speed at which a tape can be driven while maintaining precise control of the speed is below 180 inches per second. The maximum density of digital data that can be recorded on tape is 33.33.times.10.sup.3 bits per inch per track. This gives a data rate of 6 Megabits per second per track. For a 24 track recording system the maximum data rate is thus 144 Megabits per second.
The rate at which digitized video information is generated depends on the resolution of the imager. Resolution is usually measured by the number of lines per frame that the imager generates. Video imagers which adhere to the EIA RS-170 or RS-330 NTSC video formats generate video frames at the rate of 30 frames per second, regardless of the resolution of the imager. Thus, the higher the resolution of an imager, the more rapidly information is generated. A single 525 lines per frame video frame is made up of 3.149.times.10.sup.5 picture elements (pixels). At 30 frames per second this corresponds to a data rate of 9.45.times.10.sup.6 pixels per second. In a video recording system where each pixel is represented by a 10 bit-word, the data rate from a 525 line imager is thus 94.5 Megabits per second. An equivalent analysis for a 1024 lines per frame image yields a data rate of 319.49 Megabits per second. The 525 lines per frame information could be recorded directly onto the tape since the data rate is below the maximum data rate of the recorder. The 1024 lines per frame information exceeds the maximum rate of the recorder (144 Megabits per second) by a factor of more than two.
Since high resolution imagers generate information at rates higher than 144 Megabits per second, there exists a need for a digital recording system that is capable of preserving the integrity of that information throughout the recording and reproduction processes and, at the same time, slowing the data rate to the recording system.